1. Field of the Invention
The present invention relates to a miniaturized harmonic generator that can be modulated, the light source being useful in image display, image formation, optical information processing, optical recording, and optical instrumentation.
2. Related Background Art
A miniaturized visible laser-light source (wavelengths in the range of 400 to 700 nm) that can be modulated is a promising light source and will find its wide applications in the fields of display, digital photo printer, laser beam printer, optical memory, laser machine, and laser therapy equipment and other fields such as information processing, instrumentation, medical service, and biotechnology. A semiconductor laser is small and can be modulated at a high rate. However, in the range of visible wavelengths, only limited semiconductor lasers are reliable and currently in practical use. They are a red laser in the range of 630 to 700 nm, and purple and blue lasers in the range of 400 to 460 nm. This depends on the development of compound semiconductor materials that constitute a semiconductor laser. There are many problems that must be overcome before semiconductor lasers in the range of wavelengths between blue and red, i.e., blue-green, green, yellow-green, yellow, and orange can be practically applied.
A conventional miniaturized laser in the green region is a wavelength conversion laser in which a semiconductor laser of 808 nm is used as a pumping light to cause a solid-state laser made of Nd:YAG crystal or Nd:YVO4 crystal to oscillate and the second harmonic of the oscillation is used. Solid-state lasers such as Nd:YAG and Nd:YVO4 have a wavelength of 1064 nm. A non-linear crystal is used to convert the oscillated light into the second harmonic, i.e., green light having a wavelength of 532 nm. However, with the configuration in which a solid-state laser generates light having a fundamental-wavelength, the modulation rate of the laser light is limited by the lifetime of fluorescence of the solid-state crystal. For example, because the lifetime of fluorescence is 230 μsec for Nd:YAG crystal and 90 μsec for Nd:YVO4 crystal, the modulation rate is only several tens kHz at the highest. For higher modulation rates, an external light modulator based on acoustooptic effect or electrooptical effect is required at the output of the wavelength conversion laser. The provision of an external light modulator has problems that (1) the overall size and complexity of the apparatus increase, (2) energy efficiency decreases due to transmission of light through an external light modulator, and (3) driving a light modulator necessitates additional power consumption.
In order to solve this problem, “OPTRONICS” on pages 89–93, January, 2000 proposes a configuration in which semiconductor laser light is introduced into a non-linear crystal device to directly convert the wavelength. With the configuration described in “OPTRONICS”, periodical domain inversion is formed in a non-linear optical crystal to increase effective non-linear optical constant. Additionally, an optical waveguide can be used to increase power density of a fundamental wavelength. Thus, wavelength conversion efficiency is improved. However, quasi phase-matching based on domain inversion has a wavelength tolerance of 0.1 nm with respect to the fundamental wave. In order to couple laser light into an optical waveguide, the laser light needs to be focused into a small beam diameter. To meet these requirements, the wavelength and mode of oscillation in the semiconductor laser need to be stabilized during modulation. Semiconductor lasers used in the field of optical communications use a grating in the form of an optical waveguide to lock wavelength. Such semiconductor lasers are a distributed Bragg reflector laser and a distributed feedback laser. These lasers meet these requirements as a semiconductor laser used in the field of optical communication. Specifically, these semiconductor lasers include InGaAsP/InP semiconductor lasers of 1300 nm and 1550 nm and AlGaAs/GaAs semiconductor lasers of 850 nm. In other words, direct wavelength conversion of ½-wavelengths of these wavelengths can be easily accomplished and therefore modulating the fundamental wavelengths will modulate the second harmonics of the fundamental wavelengths.
However, the second harmonics in the aforementioned wavelengths of optical communications have about 430 nm, 650 nm, and 780 nm, lacking wavelengths between blue light and red light. Light in the middle of the range of visible laser (i.e., from blue-green to orange) is obtained from fundamental wavelengths from 980 nm to 1200 nm. Typical semiconductor lasers that oscillate at wavelengths from about 980 nm to 1200 NM are those that employ a strained quantum well of InGaAs as an active region. It should be noted that GaAs as a laser substrate is transparent in these wavelengths and therefore the light is reflected between an n-electrode and a p-electrode a plurality of times through the cap layer and the substrate. This causes mode hopping and multiple peaks in the oscillated spectrum. Generally, in order to obtain a high-power second harmonic, a high-power semiconductor laser is required. However, modulating a semiconductor laser with a large current will result in large fluctuations of oscillation wavelength and oscillation mode, so that wavelength conversion efficiency is not stable. Thus, it is difficult to increase modulation rate. As shown in FIGS. 8A and 8B, Japanese Patent Application Laid-Open No. 11-232680 discloses modulation in which the intensity of light is modulated so that the fluctuation of average output power of a semiconductor laser is within ±20% and the oscillation wavelength is always in an allowable range of phase-matching wavelengths, i.e. a range of wavelengths allowable as wavelength fulfilling the pulse-matching condition.
However, with the aforementioned conventional art, the average output of a semiconductor laser is controlled within a predetermined range and therefore the second harmonic output (e.g., blue output in FIGS. 8A and 8B) is always present within the wavelength tolerance of phase matching. This implies that extinction ratio cannot be large in amplitude modulation. For this reason, for the applications in image displaying apparatus and image forming apparatus, contrast ratio and gradation are considerably limited. For pulse width modulation and pulse number modulation, a output light exists between pulses, requiring some measure for increasing signal-to-noise ratio. Further, when laser safety class is to be calculated, pulses cannot be treated as typical pulses trains that are completely separated from each other. Thus, a problem is that this imposes a barrier on correct safety classification.